Field of the Invention
The present invention relates to a motor driving device that converts alternating current power, supplied from an alternating current power source side, into direct current power, outputs the direct current power to a DC link, then further converts the direct current power into alternating current power for driving a motor, and supplies the alternating current power to a motor. In particular, the present invention relates to a motor driving device that can detect abnormal heat generation in an initial charging circuit which charges a DC link capacitor.
Description of the Related Art
In motor driving devices that drive motors in machine tools, press forging machines, injection molding machines, industrial machines, or various robots, alternating current power supplied from an alternating current power source side is temporally converted into direct current power, the direct current power is then further converted into alternating current power, and the alternating current power is used as driving power for a motor disposed to each driving axis. Such a motor driving device includes: a converter (rectifier) that converts (rectifies) alternating current power supplied from an alternating current power source side to output direct current power; and an inverter (invertor) that is connected to a DC link which is in the direct current output side of the converter, and that performs power conversion between direct current power for a DC link and alternating current power which is driving power or regenerative power for a motor. The motor driving device controls the rate or torque of a motor connected to the alternating current side of the inverter, or the position of a rotor.
A DC link capacitor is disposed to a DC link that connects the direct current output side of a converter and the direct current input side of an inverter to each other. The DC link capacitor has a function as a smoothing capacitor for inhibiting the pulsating component of the direct current output of the converter, and a function as a condenser that can accumulate direct current power.
As described in Japanese Unexamined Patent Publication No. H09-140051, it is preferable to put a DC link capacitor on an initial charge (also referred to as “pre-charge”) immediately after starting up a motor driving device and before starting driving of a motor (i.e., before start of power conversion operation by inverter unit), and therefore the DC link capacitor is commonly provided with an initial charging circuit for such an initial charge.
FIG. 4 is a view illustrating the configuration of a common motor driving device. A motor driving device 1000 includes: a converter 111 that converts alternating current power supplied from a commercial three-phase alternating current power source 3 to output direct current power; and an inverter 112 that is connected to a DC link which is in the direct current output side of the converter 111, and that converts the direct current power output from the converter 111 into alternating current power supplied as driving power for a motor 2 or that converts alternating current power regenerated from the motor 2 into direct current power. The motor driving device 1000 controls the rate or torque of the motor 2 connected to the alternating current side of the inverter 112, or the position of a rotor.
In order to individually supply driving power to motors 2 disposed correspondingly to a plurality of driving axes to control driving of the motors 2, inverters 112, the number of which is equal to the number of the motors 2, are connected in parallel. Each of DC link capacitors 113 is provided for the direct current input side of each inverter 112. In other words, the DC link capacitors 113 are located in the DC link sides, to which such converters 111 are connected, of the inverters 112. As an example, in FIG. 4, the number of the motors 2 is set at three, and therefore the number of inverters 112 is three. In contrast, one converter 111 is often provided for plural inverters 112 for the purpose of reducing the cost and space occupied by the motor driving device 1000.
Large inrush current flows through the converter 111 immediately after starting an initial charge from the state of accumulating no energy in the DC link capacitor 113. In particular, the larger the capacitance of the DC link capacitor 113, the larger the generated inrush current. As a measure against the inrush current, an initial charging circuit 114 is disposed between the converter 111 and the DC link capacitor 113 in the inverter 112 in the motor driving device 1000.
The initial charging circuit 114 includes: a charging resistance 122; and a switch 121 that is connected in parallel to the charging resistance 122 to short-circuit both ends of the charging resistance 122 when the switch 121 is closed. The switch 121 is opened (turned off) only during the period of the initial charge of the DC link capacitor 113 immediately after starting up the motor driving device 1000, and is maintained in the state of being closed (turned on) during the period of normal operation in which the motor driving device 1000 drives the motor 2. More specifically, direct current output from the converter 111 is allowed to flow into the DC link capacitor 113 through the charging resistance 122 to charge the DC link capacitor 113 by opening (turning off) the switch 121 during the period of the initial charge immediately after starting up the motor driving device 1000 and before starting the driving of the motor 2. When the DC link capacitor 113 is charged to a predetermined voltage, the switch 121 is closed (turned on) to short-circuit both of the ends of the charging resistance 122 and to complete initial charge operation. Then, the inverter 112 starts power conversion operation to supply driving power to the motor 2 and to drive the motor 2 based on the driving power.
Because direct current power output from the converter 111 is allowed to flow through the charging resistance 122 and is consumed as heat in the charging resistance 122 by opening (turning off) the switch 121 during the period of the initial charge of the DC link capacitor 113, excessive inrush current is prevented from being generated during the initial charge period. However, the charging resistance 122 has a load capacity defined as a heat quantity at which fusion can be withstood, and the flow of excessive current that exceeds the load capacity through the charging resistance 122 causes the charging resistance 122 to generate abnormal heat and to be fused. Therefore, monitoring of current flowing through the charging resistance 122 and detection of abnormal heat generation are important for protecting the charging resistance 122.
For example, as described in Japanese Unexamined Patent Publication No. H09-140051, there is known a technology including a current sensor for detecting current in itself flowing through a charging resistance, in which abnormality is determined in order to protect the charging resistance when current detected by the current sensor exceeds a predetermined magnitude, during the period of the initial charge of a DC link capacitor.
For example, as described in Japanese Unexamined Patent Publication No. H08-317660, there is known a technology to monitor a potential difference between both ends of a charging resistance, and to determine abnormality in order to protect the charging resistance when a state in which the potential difference is prevented from being a specified value or less continues for longer than a given time, during the period of the initial charge of a DC link capacitor.
FIG. 5A is a view for explaining a relationship between a potential difference between both ends of a charging resistance and a potential difference between both ends of a DC link capacitor in a case in which the motor driving device illustrated in FIG. 4 normally drives a motor, and FIG. 5A indicates the potential difference between both of the ends of the charging resistance. FIG. 5B is a view for explaining a relationship between a potential difference between both of the ends of the charging resistance and a potential difference between both of the ends of the DC link capacitor in a case in which the motor driving device illustrated in FIG. 4 normally drives the motor, and FIG. 5B indicates the potential difference between both of the ends of the DC link capacitor. When the motor driving device 1000 accelerates the motor 2 between a time t1 and a time t2 using motor supply current having a magnitude suitable for the capacitance of the DC link capacitor 113 after the completion of the initial charge of the DC link capacitor 113, the switch 121 in the initial charging circuit 114 is closed to short-circuit both of the ends of the charging resistance 122. Therefore, no direct current flows through the charging resistance 122. Thus, no potential difference occurs between both of the ends of the charging resistance 122 as indicated in FIG. 5A, and a state in which the direct current voltage of the direct current output side of the converter 111 is applied to the DC link capacitor 113 on an as-is basis as indicated in FIG. 5B is achieved. Hereinafter, “motor supply current having a magnitude suitable for capacitance” means “motor supply current at which driving of motor can be accurately controlled when set DC link capacitor capacitance is used”. When the motor 2 is normally driven using motor supply current having a magnitude suitable for the capacitance of the DC link capacitor 113 as described above, no direct current flows through the charging resistance 122, and the charging resistance 122 generates no heat.
FIG. 6 is a view illustrating a motor driving device in which the switch in the initial charging circuit illustrated in FIG. 4 has failed, and the switch is opened even during the period of the normal driving operation of a motor. FIG. 7A is a view for explaining a relationship between a potential difference between both ends of a charging resistance and a potential difference between both ends of a DC link capacitor in the case of normally driving the motor using motor supply current having a magnitude suitable for the capacitance of the DC link capacitor in the motor driving device including the broken-down switch illustrated in FIG. 6, and indicates the potential difference between both of the ends of the charging resistance. FIG. 7B is a view for explaining a relationship between a potential difference between both of the ends of the charging resistance and a potential difference between both of the ends of the DC link capacitor in the case of normally driving the motor using motor supply current having a magnitude suitable for the capacitance of the DC link capacitor in the motor driving device including the failed switch illustrated in FIG. 6, and indicates the potential difference between both of the ends of the DC link capacitor. When the switch 121 in the initial charging circuit 114 has failed and thus opened as illustrated in FIG. 6 during a normal operation period during which the motor driving device 1000 drives the motor 2 after completion of the initial charge of the DC link capacitor 113, direct current flows through the charging resistance 122 even during the period of the normal driving operation of the motor 2. In such a case, acceleration of the motor 2 between a time t1 and a time t2 using motor supply current having a magnitude suitable for the capacitance of the DC link capacitor 113 results in occurrence of a potential difference between both of the ends of the charging resistance 122 as indicated in FIG. 7A, in a sharp drop in the voltage of both of the ends of the DC link capacitor 113 as indicated in FIG. 7B, and in immediate alarm-stopping of the motor driving device 1000 (time t3). Normally, a time before the alarm-stopping (i.e., between time t1 and time t3) is extremely short, and therefore abnormal heat generation does not occur in the charging resistance 122.
FIG. 8A is a view for explaining a relationship between a potential difference between both of the ends of the charging resistance and a potential difference between both of the ends of the DC link capacitor in the case of driving the motor using motor supply current that is low with respect to the capacitance of the DC link capacitor in the motor driving device including the broken-down switch illustrated in FIG. 6, and indicates the potential difference between both of the ends of the charging resistance. FIG. 8B is a view for explaining a relationship between a potential difference between both of the ends of the charging resistance and a potential difference between both of the ends of the DC link capacitor in the case of driving the motor using motor supply current that is low with respect to the capacitance of the DC link capacitor in the motor driving device including the failed switch illustrated in FIG. 6, and indicates the potential difference between both of the ends of the DC link capacitor. When the switch 121 in the initial charging circuit 114 is failed and thus opened as illustrated in FIG. 6 during a normal operation period during which the motor driving device 1000 drives the motor 2 after the completion of the initial charge of the DC link capacitor 113, acceleration of the motor 2 between a time t1 and a time t2 using motor supply current that is low with respect to the capacitance of the DC link capacitor 113 results in flow of direct current through the charging resistance 122, in generation of a potential difference between both of the ends of the charging resistance 122 as indicated in FIG. 8A, and in a sharp drop in the voltage of both of the ends of the DC link capacitor 113 as indicated in FIG. 8B. Alarm-stopping of the motor driving device 1000 does not occur, and the motor driving device 1000 carries out normal driving operation during the period of the acceleration of the motor between the time t1 and the time t2. However, occurrence of abnormality is determined when a state in which a potential difference occurs between both of the ends of the charging resistance 122 continues for a given time in the technology according to Japanese Unexamined Patent Publication No. H08-317660.
A technology such as that according to Japanese Unexamined Patent Publication No. H09-140051 has a drawback in that disposition of a current sensor that monitors current flowing through a charging resistance is necessary for detecting abnormal heat generation in a charging resistance in an initial charging circuit, and thus cost is increased.
In the technology according to Japanese Unexamined Patent Publication No. H08-317660, even when a switch in an initial charging circuit is closed to short-circuit both ends of a charging resistance during a normal operation period during which a motor driving device drives a motor after the completion of the initial charge of a DC link capacitor, a potential difference occurs between both of the ends of the charging resistance in the initial charging circuit depending on a combination of the magnitude of alternating current supplied to the motor (hereinafter simply referred to as “motor supply current”) and the capacitance of the DC link capacitor, and it is thus possible misdetection of “occurrence of abnormal heat generation in charging resistance” to occur, although no current is flowing though the charging resistance. This will be described in more detail with reference to FIG. 9A and FIG. 9B.
FIG. 9A is a view for explaining a relationship between a potential difference between both of the ends of the charging resistance and a potential difference between both of the ends of the DC link capacitor in the case of driving the motor using motor supply current that is high with respect to the capacitance of the DC link capacitor in the motor driving device including the not-failed switch illustrated in FIG. 4, and indicates the potential difference between both of the ends of the charging resistance. FIG. 9B is a view for explaining a relationship between a potential difference between both of the ends of the charging resistance and a potential difference between both of the ends of the DC link capacitor in the case of driving the motor using motor supply current that is high with respect to the capacitance of the DC link capacitor in the motor driving device including the not-broken-down switch illustrated in FIG. 4, and indicates the potential difference between both of the ends of the DC link capacitor. When the motor driving device 1000 accelerates the motor 2 between a time t1 and a time t2 using motor supply current that is high with respect to the capacitance of the DC link capacitor 113 after completion of the initial charge of the DC link capacitor 113, the switch 121 in the initial charging circuit 114 is closed to short-circuit both of the ends of the charging resistance 122, and therefore no direct current flows through the charging resistance 122, thus preventing abnormal heat generation from occurring in the charging resistance 122. However, in order for the inverter 112 to supply high motor supply current to the motor 2 so as to accelerate the motor 2, it is necessary to discharge large direct current power from the DC link capacitor 113 having a low capacitance and to supply the large direct current power to the inverter 112. Thus, the voltage of both of the ends of the DC link capacitor 113 drops as indicated in FIG. 9B. According to Kirchhoff's Law, a drop in the power of the DC link capacitor 113 appears as a potential difference between both of the ends of the charging resistance 122. As a result, misdetection of “occurrence of abnormal heat generation in charging resistance” occurs, although no direct current is flowing though the charging resistance 122 in the technology according to Japanese Unexamined Patent Publication No. H08-317660, in which abnormally is determined based on the potential difference between both of the ends of the charging resistance 122.